Forces on the nuclei of a molecule in optical fields

The electric Lorentz forces acting upon the nuclei of a vibrating molecule cause variations of dynamical regime and determine the intensity of the absorbed radiation. These forces, depending on the local electric field, can be evaluated by frequency-dependent electric and electromagnetic shielding and hypershielding tensors at the nuclei. A general expression from time-dependent perturbation theory is all that one needs to rationalize the molecular response by predicting the effective electric field at the nuclei of a molecule perturbed by an external monochromatic wave. The electric and electromagnetic hypershieldings are connected with the geometrical derivatives of the frequency-dependent dipole polarisability and of the optical rotatory power, respectively. Intensities in Raman spectroscopy and in vibrational Raman optical activity, usually interpreted in terms of these derivatives, can also be discussed via nuclear electromagnetic hypershieldings. Conditions for translational and rotational invariance can be expressed via sum rules for the dynamic hypershieldings.